At the Institute for Marine Mammal Studies in Mississippi, Kelly the dolphin has built up quite a reputation. All the dolphins at the institute are trained to hold onto any litter that falls into their pools until they see a trainer, when they can trade the litter for fish. In this way, the dolphins help to keep their pools clean.

Kelly has taken this task one step further. When people drop paper into the water she hides it under a rock at the bottom of the pool. The next time a trainer passes, she goes down to the rock and tears off a piece of paper to give to the trainer. After a fish reward, she goes back down, tears off another piece of paper, gets another fish, and so on. This behaviour is interesting because it shows that Kelly has a sense of the future and delays gratification. She has realised that a big piece of paper gets the same reward as a small piece and so delivers only small pieces to keep the extra food coming. She has, in effect, trained the humans.

Her cunning has not stopped there. One day, when a gull flew into her pool, she grabbed it, waited for the trainers and then gave it to them. It was a large bird and so the trainers gave her lots of fish. This seemed to give Kelly a new idea. The next time she was fed, instead of eating the last fish, she took it to the bottom of the pool and hid it under the rock where she had been hiding the paper. When no trainers were present, she brought the fish to the surface and used it to lure the gulls, which she would catch to get even more fish. After mastering this lucrative strategy, she taught her calf, who taught other calves, and so gull-baiting has become a hot game among the dolphins.

“Intelligence” is a term with many definitions and interpretations. It’s difficult enough to measure in humans let alone other animals. Large brains are traditionally associated with greater intelligence, and the brain of the adult bottlenose dolphin is about 25% heavier than the average adult human brain. Generally though, larger mammals tend to have larger brains, and so a more accurate estimate of brain power comes from the ratio of brain size to body size - the “encephalisation quotient” (EQ). While river dolphins have an EQ of 1.5, some dolphins have EQs that are more than double those of our closest relatives: gorillas have 1.76, chimpanzees 2.48, bottlenose dolphins 5.6. The bottlenose’s EQ is surpassed only by a human’s, which measures 7.4 (Australopithecines - hominids that lived around 4m years ago - fall within the dolphin range: 3.25-4.72). But we don’t know enough about the workings of the brain to be sure of what these anatomical measurements truly represent. Today, most scientists share the view that it is behaviour, not structure, that must be the measure of intelligence within a species.

Dolphins have invented a range of feeding strategies that more than match the diversity of habitats in which they live. In an estuary off the coast of Brazil, tucuxi dolphins are regularly seen capturing fish by “tail whacking”. They flick a fish up to 9 metres with their tail flukes and then pick the stunned prey from the water surface. Peale’s dolphins in the Straits of Magellan off Patagonia forage in kelp beds, use the seaweed to disguise their approach and cut off the fishes’ escape route. In Galveston Bay, Texas, certain female bottlenose dolphins and their young follow shrimp boats. The dolphins swim into the shrimp nets to take live fish and then wriggle out again - a skill requiring expertise to avoid entanglement in the fishing nets.

Dolphins can also use tools to solve problems. Scientists have observed a dolphin coaxing a reluctant moray eel out of its crevice by killing a scorpion fish and using its spiny body to poke at the eel. Off the western coast of Australia, bottlenose dolphins place sponges over their snouts, which protects them from the spines of stonefish and stingrays as they forage over shallow seabeds.

A dolphin’s ability to invent novel behaviours was put to the test in a famous experiment by the renowned dolphin expert Karen Pryor. Two rough-toothed dolphins were rewarded whenever they came up with a new behaviour. It took just a few trials for both dolphins to realise what was required. A similar trial was set up with humans. The humans took about as long to realise what they were being trained to do as did the dolphins. For both the dolphins and the humans, there was a period of frustration (even anger, in the humans) before they “caught on”. Once they figured it out, the humans expressed great relief, whereas the dolphins raced around the tank excitedly, displaying more and more novel behaviours.

Dolphins are quick learners. Calves stay with their mothers for several years, allowing the time and opportunity for extensive learning to take place, particularly through imitation. At a dolphinarium, a person standing by the pool’s window noticed that a dolphin calf was watching him. When he released a puff of smoke from his cigarette, the dolphin immediately swam off to her mother, returned and released a mouthful of milk, causing a similar effect to the cigarette smoke. Another dolphin mimicked the scraping of the pool’s observation window by a diver, even copying the sound of the air-demand valve of the scuba gear while releasing a stream of bubbles from his blowhole.

Many species live in complex societies. To fit in, young dolphins must learn about the conventions and rules of dolphin society, teamwork and who’s who in the group. For these dolphins, play provides an ideal opportunity to learn about relationships in a relatively non-threatening way. At Sarasota Bay in Florida, Randall Wells and his team have observed groups of juvenile male bottlenose dolphins behaving like boisterous teenage boys. Using its head to do the lifting, one dolphin may even get another dolphin air borne, actually tossing it out of the water. It’s unclear exactly what is going on. It could be play, but more likely these are serious interactions that are defining social relationships.

Dolphins gradually build up a network of relationships, ranging from the strong bond between a mother and calf, to casual “friendships” with other community members. Wells and his team were the first to notice that adult male bottlenose dolphins tend to hang out in pairs. The dolphins’ motivation for ganging together is under study but may involve ecological and/or reproductive benefits. Dolphins may also form “supergangs”. Richard Connor and his team in Shark Bay, Western Australia, discovered a group of 14 males. The supergang was a force to be reckoned with. In the three years it was studied, it never lost a fight.

To keep track of the many different relationships within a large social group, it helps to have an efficient communication system. Dolphins use a variety of clicks and whistles to keep in touch. Some species have a signature whistle, which, like a name, is a unique sound that allows other dolphins to identify it. Dolphins also communicate using touch and body postures. By human definition, there is currently no evidence that dolphins have a language. But we’ve barely begun to record all their sounds and body signals let alone try to decipher them. At Kewalo Basin Marine Laboratory in Hawaii, Lou Herman and his team set about testing a dolphin’s ability to comprehend our language. They developed a sign language to communicate with the dolphins, and the results were remarkable. Not only do the dolphins understand the meaning of individual words, they also understand the significance of word order in a sentence. (One of their star dolphins, Akeakamai, has learned a vocabulary of more than 60 words and can understand more than 2,000 sentences.) Particularly impressive is the dolphins’ relaxed attitude when new sentences are introduced. For example, the dolphins generally responded correctly to “touch the frisbee with your tail and then jump over it”. This has the characteristics of true understanding, not rigid training.

Lou Herman and Adam Pack taught the dolphins two further signals. One they called “repeat” and the other “different”, which called for a change from the current behaviour. The dolphins responded correctly. Another test of awareness comes from mirror experiments. Diana Reiss and her researchers installed mirrors inside New York Aquarium to test whether two bottlenose dolphins were self-aware enough to recognise their reflections. They placed markings in non-toxic black ink on various places of the dolphins’ bodies. The dolphins swam to the mirror and exposed the black mark to check it out. They spent more time in front of the mirror after being marked than when they were not marked. The ability to recognise themselves in the mirror suggests self-awareness, a quality previously only seen in people and great apes.

Not only do dolphins recognise their mirror images, but they can also watch TV. Language-trained chimps only learned to respond appropriately to TV screens after a long period of training. In contrast, Lou Herman’s dolphins responded appropriately the very first time they were exposed to television.

Of course, an understanding of TV is of little use in the wild, but the ability to respond to new situations has huge implications. In the shallows of Florida Bay, Laura Engleby and her team have recently discovered an ingenious fishing strategy. A number of the local dolphin groups seem to use a circle of mud to catch mullet. The action usually begins with one dolphin swimming off in a burst of speed. It then dives below the surface, circling a shoal of fish, stirring up mud along the way. On cue, the other dolphins in the group move into position, forming a barrier to block off any underwater escape routes. As the circle of mud rises to the surface, the mullet are trapped. Their only option is to leap clear out of the water and unwittingly straight into the open mouths of the waiting dolphins.

There is still much to learn about these flexible problem-solvers, but from the evidence so far, it seems that dolphins do indeed deserve their reputation for being highly intelligent.

More on dolphin intelligence

Zoologist Anuschka de Rohan produced last month’s Wildlife on One programme, Dolphins - Deep Thinkers? This piece is based on an article in the July issue of BBC Wildlife Magazine, available from newsagents or BBC Wildlife Magazine Subscriptions on 01795 414718.